PhD thesis. Studiul geomorfologic al degradărilor de teren din bazinul Racovei. Geomorphological study of land degradation within Racova Catchment

Transcription

1 Universitatea Alexandru Ioan Cuza din Iaşi Facultatea de Geografie şi Geologie Departamentul de Geografie Școala Doctorală de Geoștiințe PhD thesis Studiul geomorfologic al degradărilor de teren din bazinul Racovei Geomorphological study of land degradation within Racova Catchment Supervisor Prof. univ. dr. ing. Ion Ioniţă Doctorand Samoilă (căs. Grigoraș) Claudia

2 Content of the Romanian version 1. Introduction Geographical location History research Methodological aspects General considerations on the origin and evolution of the relief Geology Physical-geographical factors controlling relief Climatic factor Hydric factor Biologic factor Pedological factor Anthropic factor Morphological features Morphography Morphometry Relief genetic types and main landforms Structural relief Lithologic-structural plateaux Valley types controlled by geological structure Sculptural relief in the general monocline (homocline) structure Hilltops Slopes (valley-sides) Fluvial accumulation relief Alluvial plain Fluvial terraces Connection landforms Biogen relief Geomorphological zoning Land degradation Soil erosion Gully erosion Mass movement Earth falls Suffosion Landslides Distribution and peculiarities of the landslides Landslide susceptibility Sedimentation Measures for soil and water conservation Conclusions References

3 Revised content of the English summary 1. Introduction Study area Methodology Results and discussion Morphological features Relief genetic types and main landforms Structural relief Sculptural relief in the general monocline (homocline) structure Fluvial accumulation relief Geomorphological zoning Land degradation Soil erosion Gully erosion Mass movements/landslides Distribution and peculiarities of the landslides Landslide susceptibility Sedimentation Measures for soil and water conservation Conclusions References Obs. Please notice that Chapters 3, 4, 5 and 6 from the Romanian version are comprised in Chapter 4 (Results and discussion) from the extended English summary. 3

4 1. Introduction refers mostly to the history of the previous research. 2. Study area Racova Catchment is located in the southern frame of the Central Moldavian Plateau at the contact with the Tutova Rolling Hills (Figure 1). It is contiguous to Buda/Stemnic Catchment to the north, Barlad Valley to the east and Lipova, Tutova, Simila and Studinet catchments to the south and south-west. Racova Catchment, orientated in a west-east direction, is 49 km long and covers 32,908 ha where live 24,093 inhabitants. Fig. 1. Location of the study area. 4

5 From the stack of Paleozoic, Mesozoic and Tertiary sedimentary strata, Late Miocene (Sarmatian and Maeotian/Messinian) layers have outcropped due to erosion (Figure 2). Fig. 2. Geological map of Racova Catchment (processing after Jeanrenaud, 1971). Racova Catchment is sculptured in clayey-sandy and sandy-clayey formations, almost exclusively deposited in deltaic facies. These formations are mostly Upper Sarmatian (Kersonian) in age, and are entirely developed on vertical and secondary Maeotian sediments. These are erosion remnants on cineritic sandstones or sands on the highest hilltops. The thickness of the Kersonian cross-bedded deltaic strata at Pungesti, in the upper Racova, is ~250 m. The Late Miocene sedimentary strata lie in a general homocline structure, with a gentle dip of 7-8 m km -1 towards the south-east (Jeanrenaud, 1961, 1966, 1971; Ionesi, 1994). There are also recent Quaternary formations, including eluvia, diluvia, colluvia, proluvia and alluvia. The continental temperate climate is characterized by a mean annual temperature rising from 7.5 C on the highest hilltops to 9.9 C at Vaslui on the floodplain of the Barlad Valley. Mean annual precipitation for the period varied from 533 mm at Vaslui at 105 m a.s.l and 588 mm at Floreşti at 255 m a.s.l. to ~700 mm in the area >400 m a.s.l. Usually, 67-68% of 5

6 mm precipitation falls during the warm season (April-September), with the monthly maximum falling in June, with typical totals of mm (Figure 3) I II III IV V VI VII VIII IX X XI XII Floreşti+Lipova Puşcaşi Fig. 3. Mean monthly precipitation at Floresti ( ) plus Lipova and Puscasi over (processing after ABA Prut-Barlad data). Racova River is a right tributary of the Barlad Catchment. Mean monthly water discharge over shows significant pulses in connection with the climatic conditions. The peak of 0.51 m 3 s -1 occurred during April due to both snowmelt and the first spring rainfalls. The low values of m 3 s -1 are recorded at the end of summer (in August) and in October, when drought frequently strikes the area. Bio-pedo-geographically, the higher areas are covered with deciduous forest, where there is a gradual transition from common beech (Fagus silvatica) to the prevalent durmast (Quercus petraea). The sylvo-steppe is advancing from the Barlad Valley, and is composed of quercine coppices and meadows consisting mainly of fescue grasses. Accordingly, the zonal soils in the higher districts are Luvisols (Entic Luvisols prevail and alternate with Luvisols). Lower areas are predominantly Cernisols (Phaeozems and Cambic Chernozems). A large proportion of slopes is mantled by less productive soils (Erodosols and Regosols), depending on the stage of land degradation processes. Most floodplain soils are Alluviosols and Hydrisols. Agricultural land covers 68% (22,397 ha) of the catchment area. Some 35.4% is arable and 26% is pasture. The proportion of non-agricultural land is 32% (10,511 ha), of which woodland covers 26.6% (8,614 ha). This region was severely deforested during the 18 th and especially the 19 th Century. The area under native forest in Racova Catchment comprises only 6

7 17.8% (5,728 ha) and has remained fairly constant since the late 19 th Century. At present, woodland also includes the afforestation (silvic plantations) area on 5.2% of the catchment area (1,704 ha). The morphological characters of the relief within Racova Catchment are emphasized by the shading map of the study area (Figure 4). Fig. 4. The shading map of the Racova Catchment. 3. Methodology Several methods were deployed to estimate soil erosion losses, gully distribution, landslide inventory and reservoir sedimentation rates. Firstly, a Digital Elevation Model (DEM) was created by digitizing the national 1:5,000 topographic maps using TNT Mips version 6.9 software. The Universal Soil Loss Equation (USLE) developed by Wischmeier & Smith (1960, 1978) with its six factors, as adopted for Romanian conditions (Motoc, 1970; Motoc et al., 1975), was used to estimate long-term average annual soil loss from Racova Catchment. Information for present-day land use was abstracted from the 2009 aerial orthophotos and the 1:5,000 topographic maps. These sources, together with the 2012 LiDAR images, have been successfully used to draw gully outlines and especially land covered by landslides. Useful 7

8 information about land use and the gully network at the end of the 19 th Century was drawn from both the topographic map of Moldavia (scale 1:20,000) and the 1894 Atlas of Moldavia (scale 1:50,000). Levelling (topographic) surveys, using a Leica 407 TCR and GPS South 82V-Trimble, were conducted to obtain information about the behaviour of the check-dams constructed during the early 1970s along gullies within Ivanesti sub-catchment. The GPS South 82V-Trimble was used to obtain 2,081 points on the former submerged floor of Puscasi Reservoir, including the extensive but temporarily emerged area during spring Six bathymetrical cross-sections and a longitudinal profile were surveyed using the Midas Valeport Eco-sounder, type Bathy-500DF, in the permanently submerged area. A very detailed topographic map, consisting of a grid of 749 points covering the floor of the future Puscasi Reservoir, was completed in December 1969 by ISPIF (Institute for Land Treatments Studies and Designs), Bucharest. We used the map to calculate the thickness and volume of deposited sediment in the reservoir over a 44-year period. A mean bulk density of 1.45 t m -3 has been frequently used in the study area to convert volumes to sediment yield (SY) and sediment delivery ratio (SDR) at the catchment outlet. The 137 Cs technique was used along gully floors in the Ivanesti sub-catchment to estimate the impact of soil conservation measures (check dams and afforestation). Gamma spectroscopy, associated with the Canberra MCA S100 system equipped with a Ge (Li) detector, was used to determine 137 Cs concentrations in sediments. Soil surveys at 1:10,000 scale were retrieved from OSPA Vaslui (Office for Pedological and Agrochemical Surveys) to distinguish the main soil classes and types. Data processing was performed using Microsoft Office 2010 and particular attention was given to ground truthing cartographic information. 4. Results and discussion The results obtained provide insights to some morphological features, the main landforms and especially land degradation processes Morphological features The local relief is typically hilly, with extreme altitudes of m a.s.l. on Mangaralia Hill in the southern watershed and 89 m a.s.l. on the Barlad floodplain. About 14,600 ha, representing 44.4% of the catchment area, range from m a.s.l. (Figure 5). 8

9 Weight (%) < >400 Hypsometric classes (m) Fig. 5. Distribution of the altitudinal classes (m) in Racova Catchment. The overall mean slope of Racova Catchment is estimated to be 18.7%, with overall slopes of 17.8% on the left side and 20.6% on the right side. These almost even values on both sides of Racova Catchment blur the classical asymmetrical pattern of the geo-declivity, and it has resulted from severe land dissection. The highest slope values >27% are on the cuesta fronts, in a series of heavily degraded cuesta back-slopes, and in the upper catchment of the Racova tributaries. According to the slope histogram, 87% of the study area exceeds 5%, which indicates that Racova Catchment has high erosion potential (Figure 6). Fig. 6. Slope map (%) within Racova Catchment. 9

10 4.2. Relief genetic types and main landforms All major relief types typically for the platform areas have been identified, namely: structural, sculptural and accumulation relief (Figure 7). Fig. 7. Geomorphological map of Racova Catchment The structural topography is associated with structural-lithological plateaux, mainly developed on Maeotian cineritic sandstones in the shape of erosion remnants. Even though they occupy only 0.74% of the total area, their peripheral location at higher elevation is decisive in enforcing the plateau character of the area. The most representative structural-lithological plateaux are located on the southern frame of Racova Catchment at m a.s.l. in the following hills: Poienesti, Oprisita, Lacu Babei and Fantana Blanarului The sculptural (fluvio-denudational) topography within the homocline is the prevailing relief type and covers 27,353 ha, representing 83.1% of the total catchment area. It occurs as two landforms: sculptural hilltops and slopes (valley-sides). Most of the catchment is comprised of slopes (25,777 ha, 78.3% of Racova Catchment). They usually play the role of 10

11 cuesta front and/or cuesta back-slope underlining noticeable extension and development of the cuesta relief in the study area. Upon closer examination, however, Ionita (1998, 2000) emphasized the impact of a double-dipping system of the outcropping strata on the relief of cuestas within the Moldavian Plateau. Firstly, the main southward dip of ~6 m km -1 underlines the first order morpho-structural asymmetry which comprises all subsequent valleys. These are either west-east or east-west oriented, and show classical cross asymmetry: a steep, short north-facing cuesta front and broad, gently sloping south-facing cuesta back-slope. For example, we mention the subsequent Racova valley and the diagonal (askew) subsequent Garceneanca and Harsova valleys. Then, the lower eastward dip of ~3 m km -1, induced by the more intense tectonic uplifting at the contact with the Carpathian Orogen, resulted in the second order morpho-structural asymmetry. This includes most consequent and reconsequent valleys (e. g. Racovita, Toporasti, Spia, Cosesti, Valea Oanei, Valea Mare, Dumbrava, Recea, Dumbravita, Valea Targului, Valea Hopului, Cristoaia and Valea Hasculet) and some more developed obsequent valleys (e.g. Ivanesti and Brosteni) which contain smaller cuestas with west-facing fronts. Overall, the subsequent Racova valley shows a cross asymmetry, but not a typical one, except for the upper catchment. Within the middle and most of the lower catchment, from Blesca to Puscasi, there is a balance between the left side (53% of the total) and the right side with the remaining 47%. Here, the large north-facing cuesta front (the right valley-side) has 340 m amplitude, and consists of two steps. The lower step has a series of triangular faces of ~100 m amplitude and is situated at the base of the right valley side. The second step has a greater amplitude of ~240 m and is well defined in the upper catchment of the obsequent tributaries, especially on cineritic sandstones of the Lower Maeotian (Figure 8). Generally, the left side of Racova Catchment covers 61% of the catchment, compared with the regional norm of 70-77%. This characteristic is explained by the faster rate of southward homoclinal shifting of the northern neighbouring Stemnic/Buda valley. The asymmetrical pattern of landforms is highlighted by the broad extension of cuestas developed within the tributary valleys, such as reconsequent (Samoila & Ionita, 2017). Therefore, the background belongs to the first order structural asymmetry revealed by Racova valley, while details are provided by the second order structural asymmetry, as emphasized by most of the tributary valleys. 11

12 Fig. 8. Geomorphological cross-section through the middle Racova Catchment between Magura Hill and Cheia Hill (Samoila Claudia & Ionita, 2017). Cuesta fronts cover 15,983 ha, representing half (48.6%) of the total catchment area. The general northern looking cuesta fronts occupy 9,325 ha (28.4% of the total catchment) and the remaining 6,658 ha (20.2%) belong to the western facing cuesta fronts (Figures 9 & 10). Thus, there is a very high propensity to the land degradation processes. Fig. 9. The right valley-side (north-eastern facing cuesta front) of Racova valley upstream of Trohan (02 June 2015). 12

13 Fig. 10. The left valley-side (typical west facing cuesta front) of Racovita valley (Photo Ionita I., 20 October 2013). The cuesta back-slopes extend over 9,795 ha (29.8% of the total area), of which some 10.42% is slightly-moderately degraded and 19.34% is severely degraded The fluvial accumulation topography covering 16.1% (5,309 ha) is represented by floodplains (11.8%), fluvial terraces (0.7%) and connection landforms, glacises especially (3.6%). The scarcity of gravels, typical for the entire Moldavian Plateau, and the high relief dissection of Racova Catchment resulted in both the low extension and the poor preservation of the fluvial terraces. Besides those three levels of m, m and m rel. alt. identified by Ploscaru D. (1973), we mention a higher terrace of m a. r Geomorphological zoning Based on their geomorphologic features, three distinct compartments/sections have been distinguished, namely: western, central and eastern compartments (Samoila & Ionita, 2017). The western compartment including the upper Racova Catchment, upstream of Bleşca, with an area of 11,006.7 ha (33.45% of the total) is fully developed in Upper Sarmatian (Kersonian) clayey-sandy layers. In this section, formed by the regressive evolution of streams, Racova valley is diagonally (askew) subsequent. The broad cuesta back-slope occupies 77% of 13

14 the catchment, while the younger cuesta front is narrow, slightly dissected and its amplitude does not exceed 160 m. The central compartment, between Blesca and Puscasi, comprises the middle Racova Catchment and most of the lower catchment. This is the most representative section that occupies 19,495 ha (59.2% of total area). It is sculptured in prevailing Kersonian formations, but the subordinate Maeotian layers are largely responsible for the peculiar features of the area. Even the Racova valley is typically subsequent, there is a balance between the left side (53% of the total catchment) and the right side with the remaining 47%. In a broad sense, the right side is an ample (ca. 340 m), north facing double cuesta front (consisting in two steps of 100 and ca. 240 m), heavily dissected by obsequent tributaries. The left valley side (southern facing cuesta back-slope) of Racova exhibits a steep basal border (scarp), of ca m amplitude, which often does not represent a terrace front. We consider that this valley side foot scarp is associated to the evolution of the obsequent tributaries from the Racova cuesta front. They delivered abundant alluvia deposited along the Racova floodplain, contributing to the leftward migration of river channel and, implicitly, to the longterm undermining of the left valley side foot. The eastern compartment covers the end of the lower catchment, occupying 1,905.4 ha (5.8% of the total catchment area). The geological formations here belong only to the Kersonian, the Maeotian being eroded. The Racova valley is cross-cut subsequent, with the cuesta front associated to its narrow right valley side, of smaller amplitude ( 160 m), unitary and amounting to 46% of the section. The left valley side, southern facing cuesta back-slope is 54% of the total area and remains fairly even and slightly dissected by small re-consequent valleys. This young compartment was formed by progressive evolution, respectively, by the slight increase in length of Racova in the area of the fluvial terraces from the right side of the Barlad Valley, which slightly migrated eastwards. 14

15 4.3. Land degradation Land degradation has been recognized as the major cause of environmental degradation worldwide and, in particular, in the Moldavian Plateau of eastern Romania. The Racova Catchment is highly susceptible to soil erosion, gullying and landslides, which damages the local landscape by depleting soil resources and decreasing agricultural productivity Soil erosion Soils in the study area are highly eroded (Figure 11). Fig. 11. High and severe soil erosion on the left side of Harsova valley, downstream of Rasnita (21 April 2016). A map of soil losses from agricultural land, with five erosion classes, was created by using the USLE as adapted to Romanian conditions (Figure 12). The classes between 7-25 t ha -1 y -1 contribute an estimated 46% of the total, and one-third of soil loss is assigned to classes 25 t ha - 1 y -1 (Figure 13). The mean estimated soil loss by water erosion (rill and inter-rill) on agricultural land is 21.6 t ha -1 y -1 and this area delivers an estimated t y -1. Adjusting for the sediment contribution from woodlands, the mean specific sediment yield (SSY) decreases to 15.6 t ha -1 y -1, but still remains high. 15

16 Fig. 12. Map of soil losses from agricultural land in Racova Catchment. 21% 21% <7 t/ha/yr 7-15 t/ha/yr 12% 20% 26% t/ha/yr t/ha/yr >35 t/ha/yr Fig. 13. Weight (%) of the soil erosion classes in Racova Catchment Gully erosion Gully erosion is much more limited on the Central Moldavian Plateau, due to more erosion-resistant substrata and forest cover, compared with other subunits on the Barlad Plateau. 16

17 The most numerous gullies from Racova Catchment are discontinuous and occur especially on slopes and sometimes along valley-bottoms. The continuous gullies frequently develop on valley-bottoms and their size is very apart from the discontinuous ones (Figure 14). Fig. 14. Continuous gully Gologofta in the Ivanesti sub-catchment (29 November 2016). In Racova Catchment the present total gully length is 367 km and thus gully density is 1.12 km km -2 (Figure 15). Fig. 15. Map of gully density in Racova Catchment. 17

18 Gully area/catchment area The high value, exceeding 1.0 km km -2, typifies two distinct gully strips, namely: one located in the upper catchment of the left tributaries (particularly on the cuesta fronts) and the other on the Racova Cuesta between Ivanesti and Poienesti. These values are double those of the late 19 th Century. However, at that time, the heads of the main gullies were located close to the watershed and today they enter mostly as historical gullies (Poesen, 2011, Poesen et al., 2003). That means former road gullies developed very rapidly after deforestation. The striking low value of gully density 0.5 km km -2 on the right side of Racova valley, upstream of Blesca, is due to large forest extension on that cuesta front. By combining information from LiDAR images, ortophotoplans delivered in 2009 and field observations, it was estimated that the area covered by gullies is small (2.7% of the total, 871 ha). However, they play important roles both in the triggering or reactivation of landslides and sediment detachment and transport. Strong relationships have been established by relating the ratio gully area/catchment area versus slope classes or land orientation (Figure 16) y = x R² = 0.98 < > 27 Slope classes (%) Fig. 16. Relation between the ratio gully area/catchment area and slope classes. 18

19 Mass movement, also known as mass wasting, is the geomorphic process by which solid material move downslope largely under the force of gravity. Here are broadly included earth fall (landslip), suffusion and landslides Distribution and peculiarities of the landslides By far, landslides represent the most representative geomorphic process contributing to land degradation within Racova Catchment. The landslide inventory and map (Figure 17) shows that landslides are highly variable in size, age, shape and form, and total 56.2% (18,510.4 ha) of the catchment area. This is the highest identified proportion in the entire Moldavian Plateau and is 9% more than the mean value reported for three catchments in the Central Moldavian Plateau (Ionita et al., 2014). Fig. 17. Landslide distribution on Racova Catchment. Most landslides are stable (dormant) and the active ones form only ~3% of the total landslide area (TLA), which is a typical value for the Central Moldavian Plateau. However, after the rainier period, active landslides occupied 21.4% of the TLA (Ionita et al., 2014). 19

20 The decline in landslide activity resulted from both lower precipitation totals since 1982, and the impact of soil conservation measures deployed during the 1970s and 1980s. Most new landslides occur by local reactivation of areas that have previously experienced landslide activity (Figure 18). Fig. 18. Superficial reactivation, wave like in shape of an old landslide at the source of the Racova River (20 October 2013). Another major feature (peculiarity) of the landslides within the study area refers to formation and development of the landslide amphitheatres, locally called hartoape. These are bounded at the upper part by a semi-circular and well defined scarp and almost all the sliding deposit is moving and converging on the axis of the amphitheatre. Based on field observations it was possible to distinguish both simple and compound landslide amphitheatres, such as in the middle and upper Ivanesti sub-catchment. Within ca. 845 ha, three successive generations of landslide amphitheatres have been identified, the second one being the most impressive and consisting in a garland of five simple hartoape (Figure 19). The Landslide-Hypsometry Index (LHI) (i.e. the ratio of landslide area (LA) to total catchment area (CA) on hypsometric classes) shows a slightly asymmetrical distribution. The LHI peak value of 0.73 is typical of the m contour interval (Figure 20). By crosschecking the landslide map with the slope map, it is evident that the three main slope classes (5-18, and 27%) amounts to one-third each of the LA (Figure 21). This finding underlines 20

21 the even distribution of slope values within both sides of Racova Catchment. Three-quarters of landslides develop on cuesta fronts with the remainder occurring on degraded cuesta backslopes, especially in the upper sub-catchments. Most of the landslides are shallow and the deep seated landslides are more frequently initiated in the Kersonian strata, especially within some landslide amphitheatres. Fig. 19. Successive generations of landslide amphitheatres (hartoape) in the middle and upper Ivanesti sub-catchment. 21

22 Weight (%) Landslide area/catchment area < > 400 Hypsometrical classes (m) Fig. 20. The Landslide-Hypsometry Index (LHI) from Racova Catchment < > 27 Slope classes (%) Fig. 21. Weight of the landslide area on slope classes in Racova Catchment Landslide susceptibility represents the spatial probability or propensity of a site or area to produce landslides based on the presence of known causative factors (Crozier and Glade, 2005). Analysis of the landslide probability density on hypsometric classes from Racova Catchment suggests that the m and m classes enter with higher values of 32.1 and 28.5%. As to the relation with land slope, the peak values of the landslide probability density, of 32 and 30%, are specific to slopes exceeding 18%. The land orientation underlines 22

23 once again the highest spatial propensity to landslides of the cuesta fronts, respectively of the north-western, western, northern and north-eastern facing slopes. The final susceptibility map indicates a good correlation with the landslide inventory if compared with both the calibration subset and validation one (Figure 22). This finding is revealed by the percentage of correct ranking from the confusion matrices of 83.0%. About 28.6% (9,412 ha) of the total catchment area is included in the very high susceptibility class, 36.1% in the high class, 15.1% in the moderate class and 12.7% in the reduced susceptibility class. The value of the area under the Relative Operating Characteristic- ROC curve is 0.915, showing both the high sensivity of the logistic model and a positive rate associated with satisfactory precision of the obtained results. Fig. 22. Map of landslide susceptibility as drawn by using the logistic regression Sedimentation represents another geomorphic process with special implications on the local environment and targets floodplain aggradation and reservoir siltation, especially. In the context of more precipitation over 1968-March 1973, three dams were built in the study area, namely: Puscasi, Pungesti-Garceni and Trohan. They became operational in 1973, 1976 and 1982, respectively. 23

24 Puscasi Reservoir, in the lower Racova Catchment, is the most important (25,056 ha drainage area) and has multiple purposes: attenuation of the flood waves, water supply of Vaslui town, fishing and leisure. The other reservoirs, Pungesti on Garceneanca floodplain and Trohan in the upper Racova Catchment, are associated with small catchments (3,363 and 2,098 ha, respectively) and higher proportion of forest (32 and 44%, respectively). The map of sediment distribution on the Puscasi Reservoir floor synthetizes the deployed efforts to accurately estimate deposition through field measurements, especially using the GPS. The mean sedimentation rate is 4.7 cm yr -1 after 44 exploitation years (Figure 23). Deposition is assessed in eight classes and sediment thickness is uneven and adopts a deltaic shape. Fig. 23. Sediment distribution in Puscasi Reservoir, deposited over (Samoila Claudia et al., 2018). The mean sediment thickness (STH) deposited in Puscasi Reservoir is 206 cm and is greater in the western-half of the reservoir, and varies from m, with a peak sedimentation rate of 9 cm y -1 over 44-years. A 137 Cs profile in the area of high sedimentation showed a peak sedimentation rate of 11.5 cm y -1 during (Ionita et al. 2000). The area exceeding 2.5 m STH highlights the alluvial fan of the Racova River, which is the main 24

25 contributor to siltation in Puscasi Reservoir. However, the lateral input of close tributaries must be considered, since sediment discharge from upstream tributaries (such as the Tulbure, Ivanesti and Oprisita from the Racova cuesta front) also represents major sediment source. An exception is the low alluvia input from densely forested sub-catchments, such as the Chelaru located upstream of the right shoulder of the dam, where forest covers 61% of the sub-catchment area. The initial area of Puscasi Reservoir at normal retention level (NRL) of 257 ha has decreased by 32.3% to its current area of 174 ha, while water storage capacity has decreased by 38.6%, from m 3 to m 3. The estimated volume of sediment within Puscasi Reservoir is m 3, which represents a sediment yield (SY) of t. Estimated gross erosion (GE) of 24.8 t ha -1 y -1 is associated with the 25,056 ha catchment area (without 5,461 ha of the Pungesti-Garceni and Trohan reservoir catchment areas). Over 44 years, this totals t and thus the estimated sediment delivery ratio (SDR) is During field measurements carried out in spring 2018, when Pungesti Reservoir was temporarily almost emptied, three soil profiles were dug and sampled for Cs-137 analyses (Figure 24). Fig. 24. Soil profile in recent sediments and field measurements in Pungesti Reservoir (11 May 2018). Figure 25 illustrates that the peak value of Cs-137 from one profile occurs at cm and, therefore, the mean sedimentation rate over 32 years ( ) is 2 cm yr -1. The Cs

26 values decrease between cm depth and a revival of the Cs-137 content is noticeable in the top 30 cm. This distribution resulted from implementing the provisions of Act No. 18/1991, since the contour farming has been replaced by the traditional up-and-down slope farming under small plots. Overall, the Cs-137 depth profile is very similar to that reported by Ionita et al. (2000) located in the same spot, but the mean sedimentation rate over 12-years ( ) was surprisingly higher, 3.2 cm yr -1. The decreasing siltation rate in the Pungesti Reservoir (estimated at ca. 1 cm yr -1 since 1998) depends on increasing the share of both abandoned land and the high degree of weeding. Therefore, the mean sedimentation rate is moderate in smaller reservoirs (Pungesti-Garceni and Trohan), typical for the Central Moldavian Plateau, and high in the Puscasi Reservoir which resembles values from the Tutova Rolling Hills. Fig. 25. Depth profile of Cs-137 in the Pungesti Reservoir on 31 March 2018 (Samoila Claudia et al., 2018 & 2019) Measures for soil and water conservation By 1960, the traditional agricultural system on the hills of the Moldavian Plateau consisted of up-and-down-slope farming, with ~90% of agricultural land divided into small (<1 26

27 ha) plots. Except in local areas, there was no concern about soil erosion and little awareness of conservation practises. After 1960, these areas were turned into co-operative farms. The remaining larger plots (~10% of the area) were transferred from farmers to State farms. After several decades of quiescence, many new, innovative research studies on soil erosion control were initiated (Motoc et al., 1975 & 1992; Nistor & Ionita, 2002; Ionita et al., 2006). The first priority consisted of implementing one or more conservation practises starting with contour ploughing. By late 1989, 75% (0.9 x 10 6 ha) of agricultural land at risk of erosion on the Moldavian Plateau were adequately treated with conservation measures. The new legislation (No. 18/1991 of the Agricultural Real Estate Act) includes two provisions that discourage soil conservation measures (Motoc et al., 1992; Nistor & Ionita, 2002; Ionita et al., 2006 & 2015). One of these stipulates that land reallocation must conform to the old locations; that is plots must be orientated up-and-down slope. The second refers to the successors land rights, which apply up to the fourth degree of kinship. Under these circumstances, land division increased and it is now higher than before World War II. The major effect of the new law is the revival of the traditional agricultural system of up-and-down slope farming. Another problem over recent decades is that the State ceased funding soil conservation, and investment in soil conservation has low priority among landowners. Land changes within Racova Catchment mirrored the general changes within the entire Moldavian Plateau. During the 20 year period ( ) much soil conservation work was accomplished, especially by IEELIF Vaslui (Enterprise for Performing and Exploiting the Land Improvement Works), namely: - The design and construction of dams and reservoirs, as already mentioned. - Design and construction of check-dams to control gully erosion in the tributaries of the River Racova. - Design and implementing soil conservation practises on slopes in large farms, namely: strip-cropping, buffer strip cropping and especially bench terraces. - Design and construction of the agricultural road network. - Design and building drainage systems. - Filling small gullies, land reshaping using topsoil and improving pastures. - Large-scale afforestation on 1,704 ha on landslides and gullies, especially using black locust (Robinia pseudoacacia) and populus. Some 1,001 ha of the 27

28 afforested area were established by the Vaslui Silvic Enterprise and 703 ha by IEELIF Vaslui. After implementing the provisions of Act No. 18/1991, the former short-lived contour farming system almost disappeared and the up-and-down slope farming under small plots is prevalent again (Figure 26). The case of the 956 ha Ivanesti sub-catchment, located on the main Racova cuesta, illustrates this surprising evolution. Thus, a combination of contour strip cropping systems and bench terraces were implemented on ha of arable land, especially west of Ivanesti village. Here, on a field of 49.1 ha with an average slope of 13.6%, five m long bench terraces, spaced at m intervals, were combined with six strip crops, each covering ha (Figure 27). Fig. 26. The traditional up-and-down slope farming under small plots on the right side of the Caselor-Poienesti Valley (04 April 2016). 28

29 Fig. 27. Map of the conservation practises implemented between in the Ivanesti catchment (Samoila Claudia et al., 2018). Currently, the same field comprises 135 small individual plots, most of them orientated upand-down slope (Figure 28). About half of them are between two former bench terraces and their mean area is 0.37 ha. Others cross two, three or four former strip crops and cover ha each. Only nine plots are still on the contour and these occupy ha. Similar examples can be readily identified throughout Racova Catchment. 29

30 Fig. 28. Present-day farming system on the left side of the Ivanesti Valley (24 April 2017). In 1974, 14 check-dams were constructed along valley bottom gullies, namely: six in Canepa gully, five in Balica gully and three in Gologofta gully. They were set apart on reaches with similar original slopes of % but with various lengths, namely: 805 m in Canepa, 811 m in Balica and 310 m in Gologofta gully. Simultaneously, afforestation on 15.1% of the total area (144.3 ha) has been deployed along the gully network and especially in areas with landslides. Under these conditions, most streams are no longer competent to scour the bed or to undermine gully walls. The backwater effect of check-dams and the progressive impact of vegetation cover on stream flow decrease flow velocity and accelerate sediment deposition on the gully floor. In these cases, reducing bed gradient and the creation of trapezoidal crosssections resulted in major changes in gully morphology. Overall, gully depth decreased by m and gully bottom width increased between m relative to the original (restored) situation from Most dam structures are still in an acceptable condition of work after 44-years, except for one check-dam where the stilling basin is destroyed. The progressively combined influence of conservation measures on sedimentation rates is emphasized by the depth distribution of 137 Cs along the bottom of Balica gully. Figure 29 shows the site of a 235 cm deep alluvial profile, located 114 m upstream of check dam and the gully 30

31 cross-section. The 137 Cs peak value of 167 Bq kg -1, associated with the Chernobil accident of April 1986, occurs at cm (Figure 30). This indicates a mean sedimentation rate of 3.8 cm yr -1 over a drier 30-year time-span ( ). That rate was double between 1963 (peak year of nuclear weapons tests) and 1986, due to more precipitation (Ionita & Margineanu, 2000; Ionita et al., 2015). Therefore, almost the entire column of sediment in the gully bottom was deposited after implementing conservation measures, especially during the early 1970s. Thus, the mean sedimentation rate over 42-years ( ) is estimated to be 5.2 cm y -1 (Samoila Claudia et al., 2018). Fig. 29. Recent alluvial infilling along the bottom of Balica gully (23 November 2016). 31

32 Fig. 30. Cs-137 depth profile in recent alluvia deposited in the Balica gully (23 November 2016). 5. Conclusions The 32,098 ha Racova Catchment on the Central Moldavian Plateau is highly susceptible to land degradation, due to both natural conditions and human impacts. The local relief is typically hilly and about 14,600 ha, representing 44.4% of the catchment area, range from m a.s.l. The area in excess of 5% slope comprises 87% of the total, which indicates that Racova Catchment has high erosion potential. The almost even mean slope values on both sides of subsequent Racova Valley blur the classical asymmetrical pattern of the geo-declivity, and it has resulted from severe land dissection. Despite of being expand on only 0.74% of the total catchment area, the structurallithological plateaux underline the plateau character of the area. They have peripheral location at higher elevation ( m a.s.l.) on Maeotian cineritic sandstones as erosion remnants, mostly on the southern frame of Racova Catchment. The sculptural (fluvio-denudational) topography within the homocline is the prevailing relief type and covers 27,353 ha, representing 83.1% of the total catchment area. It occurs as 32

33 sculptural hilltops and slopes (valley-sides). Most of the catchment is comprised of slopes (25,777 ha, 78.3% of Racova Catchment). They usually play the role of cuesta front and/or cuesta back-slope underlining noticeable extension and development of the cuesta relief in the study area. Cuesta fronts cover 15,983 ha, representing half (48.6%) of the total catchment area. The general north facing cuesta fronts cover 9,325 ha and the west facing cuesta fronts cover 6,658 ha (28.4 and 20.2%, respectively) of the total. Thus, a very high propensity to the land degradation processes results. The scarcity of gravels, typical for the entire Moldavian Plateau and the high relief dissection of Racova Catchment resulted in both the low extension and the poor preservation of the fluvial terraces. Based on their geomorphologic features, three distinct compartments/sections have been distinguished (western, central and eastern), the most specific section being the central one. The mean value of soil losses by water erosion (rill and inter-rill) on agricultural land is estimated to be 21.6 t ha -1 y -1. By adding the woodland contribution, this value decreases to 15.6 t ha -1 y -1 and remains problematically high. The average gully density is 1.12 km km -2 and higher values exceeding 1.0 km km -2 typifies two distinct gully strips, namely: one located in the upper catchment of the left tributaries (particularly on the cuesta fronts) and the other on the Racova Cuesta between Ivanesti and Poienesti. Area under gullies is relatively small (870.9 ha and 2.7% of the total) but gullying plays important roles, both in the triggering or reactivation of landslides and sediment detachment and transport. The most characteristic feature of the study area is the large proportion of land (56.2%) covered by landslides, which represents the highest identified value in the entire Moldavian Plateau. Another major feature of the landslides refers to the formation and development of the landslide amphitheatres, locally called hartoape. Most landslides are shallow and the deep seated landslides occur more frequently within some landslide amphitheatres. The mean sedimentation rate is moderate (2-3 cm y -1 ) in smaller reservoirs (Pungesti- Garceni and Trohan) typical for the Central Moldavian Plateau. In turn, it is high (4.7 cm y -1 ) in the Puscasi Reservoir, which resembles values from the Tutova Rolling Hills and the estimated associated sediment delivery ratio is

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